Reversibility and biological machines

THE application of the traditional, macroscopic arguments of thermodynamics to very small systems (for instance, using individual cross bridges to generate force in striated muscle, which use "one molecule of ATP at a time"¹), has been questioned². It is demonstrated here that they cannot be applied to very small systems without very drastic modification. That is because of the requirements of quantum theory and information theory. The modifications are substantial and inescapable for systems such as individual cross bridges although they become negligible for macroscopic systems.